Securing and electrical connection of electronic parts in electronic equipment as performed when electronic parts are mounted on a printed circuit board are generally carried out by soldering, which is the most advantageous method from the standpoints of cost and reliability.
Methods which are generally employed for this type of soldering are the flow soldering method in which soldering is carried out by contacting a printed circuit board and electronic parts with molten solder, and the reflow soldering method in which soldering is carried out by reheating solder in the form of a solder paste, solder preforms, or solder balls in a reflow furnace.
In such soldering, a flux is employed as an auxiliary agent in order to facilitate adhesion of solder to a printed circuit board and electronic parts. Flux performs many actions including the following (1)-(3).
(1) Metal surface cleaning: Flux chemically removes oxide films on the metal surfaces of a printed circuit board and electronic parts, thereby cleaning the surfaces so that soldering is possible.
(2) Preventing re-oxidation: During soldering, flux covers a metal surface which has become clean and isolates the surface from contact with oxygen to prevent re-oxidation of the metal surface due to heating.
(3) Surface tension reduction: Flux reduces the surface tension of molten solder and increases the wettability of the metal surface by molten solder.
Flux can be generally classified as inorganic fluxes which use a metal salt or an inorganic acid, water soluble fluxes which use a water soluble compound such as polyethylene glycol as a base component, and resin-based fluxes which use a resin such as rosin as a base component. In addition to a resin which is used as a base component, resin-based fluxes contain at least one activator selected from organic acids, organic acid salts of an amine, and hydrohalide salts of an amine, and optionally a solvent as required to facilitate application and transfer of the flux.
Inorganic fluxes and water soluble fluxes are highly corrosive. Therefore, after soldering, it is necessary to perform cleaning in order to remove any remaining flux residue.
With a resin-based flux, a resin such as rosin which is contained in the flux as a base component is electrically insulating. After soldering by heating, flux residue, the main component of which is derived from the resin which is the base component of the flux, remains. This flux residue has good electrically insulating properties at room temperature and is not corrosive, and it covers and protects soldered portions. Therefore, it is possible to use a resin-based flux without performing cleaning.
However, in an environment with a high temperature close to 100° C. and a high humidity, the flux residue softens and becomes semi-liquid. In such a state, soldered portions are uncovered and humidity corrodes the solder, so the insulation resistance deteriorates. Therefore, a printed circuit board which is mounted on a part such as an automotive part which has the possibility of being exposed to a high-temperature, high-humidity environment is typically subjected to conformal coating with a silicone resin, an acrylic resin, an epoxy resin, or the like after the printed circuit board is cleaned after soldering. In this case, the presence of flux residue worsens the adhesion of the resulting coating, so the printed circuit board is cleaned after soldering.
Although a resin-based flux has the problem that the insulating properties of flux residue deteriorate in a high-temperature environment, flux residue itself is essentially harmless. Therefore, a no-clean type flux which does not require removal of flux residue by cleaning has been developed and actually used. Of course, a no-clean flux is preferable from the standpoint of decreasing the burden on the environment, and it is also advantageous from a cost standpoint. However, a no-clean flux often has low activity.
In both flow soldering and reflow soldering, soldering is increasingly being carried out in a nitrogen atmosphere. Oxidation does not readily take place in a nitrogen atmosphere, so a flux having low activity can be used. Accordingly, soldering in a nitrogen atmosphere is particularly suitable for no-clean fluxes.
Two types of fluxes are used in soldering of printed circuit boards by flow soldering, namely, a preflux which is applied to a circuit board immediately after it is manufactured in order to protect the surface of copper foil interconnection on the printed circuit board, and a post flux which is applied to portions to be soldered at the time of soldering either before or after mounting of electronic parts on the printed circuit board. Preflux usually does not contain an activator.
JP H05-212584 A proposes a solder paste comprising a flux and a solder powder in which an acid phosphate ester which is a dialkyl acid phosphate or a mixture of it and a monoalkyl acid phosphate is included in an amount of 5-40 parts by mass with respect to 100 parts by mass of flux. That solder paste is of a type in which flux residue remaining after soldering is cleaned off with an organic solvent. In addition, the solder powder is not a lead-free solder.
JP 2006-181635 A discloses a solder paste formed by mixing a lead-free solder powder and a flux containing an organic halide, in which the flux contains an organophosphorus compound in order to prevent blackening of soldered portions caused by the organic halide. Phosphates which are exemplified as the organophosphorus compound are neutral phosphates such as a tri-alkyl or -aryl phosphate.
Solders which have been commonly used from long in the past have been Sn—Pb alloys and particularly a Sn—Pb eutectic alloy (Sn-37Pb, melting point of 183° C.), which is simply called eutectic solder. However, due to the harmfulness of Pb, the use of Pb is now regulated on a global scale. Therefore, the use of lead-free solders which do not contain any Pb is recommended in soldering of printed circuit boards.
Lead-free solder is typically a Sn based alloy having Sn as a main component. Lead-free solders which have been proposed thus far include Sn—Ag—Cu based alloys, Sn—Cu based alloys, Sn—Ag based alloys, Sn—Bi based alloys, Sn—Zn based alloys, and the like. Except for Sn—Bi based alloys which have an extremely low melting point of at most 150° C., almost all of these lead-free solders have a high Sn content of at least to 90 mass %.
In this description, a Sn—Ag—Cu based lead-free solder, for example, means a Sn—Ag—Cu ternary alloy and alloys in which minute amounts of added element or elements are added to this ternary alloy. The same is true with respect to other “ . . . based” lead-free solders.
One lead-free solder which is currently actually used is Sn-3Ag-0.5Cu alloy (solidus temperature of 217° C. and liquidus temperature of 220° C.), but it has a soldering temperature of roughly 40° C. higher than that of the eutectic solder.
Patent Document 1: JP H05-212584 A
Patent Document 2: JP 2006-181635 A